1. Field of the Invention
The present invention relates to an image forming apparatus.
2. Description of the Related Art
An electrophotographic image forming apparatus widely employed in a copying machine, a laser printer, a facsimile and the like is equipped with a fixing device of a heat fixing system. The fixing device of a heat fixing system comprises a fixing roller and a pressure roller. The fixing roller and the pressure roller each comprise a core metal comprising a metal or the like, and an elastomer layer comprising a resin or a rubber, having excellent releasability, formed on a surface of the core metal. A heater as a heat source is provided inside the fixing roller, and the heater heats the fixing roll and the pressure roller so as to reach a given temperature by application of heat generated by the heater. In the fixing device of a heat fixing system, the fixing roller and the pressure roller are heated to a given temperature, and the heated fixing roller and pressure roller are then press-contacted with each other. As a result, when a recording medium having an unfixed toner image borne thereon passes through a fixing nip region formed between those rollers, those rollers fuse a toner constituting the unfixed toner image by application of heat, thereby fixing the unfixed toner image to the recording medium. The fixing device of a heat fixing system holds the whole of the fixing roller and the pressure roller at a given temperature, and is therefore suitable for high speed printing.
However, the fixing device of a heat fixing system is required to heat the fixing roller and the pressure roller to a given temperature by a heater provided inside the fixing roller at the time of power activation of an image forming apparatus, and this relatively increases the time of from the power activation to a printable state (warming-up time). For this reason, there is the problem that the time of from power activation by a user to output of a recording medium having an image printed thereon becomes long.
A method for shortening a warming-up time includes a method of decreasing heat capacity of a fixing roller by, for example, decreasing a thickness of a core metal of the fixing roller. A method of shortening the time until a recording medium having an image printed thereon is outputted includes a method of initiating a paper-passing operation at a fraction of the time after reaching a surface temperature of the fixing roller to a given fixable temperature.
However, the fixing device equipped with a fixing roller having small heat capacity involves the case that storage of heat of the fixing roller is not sufficient just after completion of the warming-up. In such a case, heat on the surface of the fixing roller is absorbed by the recording medium passing through the fixing nip region at the time of fixing operation, and the surface temperature of the fixing roller cannot be maintained at a given fixable temperature, leading to occurrence of poor fixing.
To achieve energy saving by reducing consumed power as much as possible, the time until moving from a standby mode (ready state) that performs heat retention of the fixing roller during stopping job to a sleep mode that does not perform heat retention of the fixing roller tends to be shortened. For this reason, in some cases, storage of heat of the fixing roller may be not sufficient just after completion of sleep return operation that returns to a job operable state from a sleep mode. Even in such as case, poor fixing occurs similarly as described before.
To overcome the problem, Japanese Unexamined Patent Publication JP-A 2004-126191 discloses an image forming apparatus in which storage of heat of a fixing roller is estimated and idle time of the fixing roller is changed after completion of warming-up according to the estimated storage of heat.
According to the image forming apparatus disclosed in JP-A 2004-126191, the period until completion of warming-up operation from power activation is measured, and storage of heat of the fixing roller is estimated based on the measurement result.
Japanese Unexamined Patent Publication JP-A 2006-119194 discloses an image forming apparatus in which time interval of conveying a recording medium to a fixing roller is prolonged or conveying speed of a recording medium is decreased. According to the image forming apparatus disclosed in JP-A 2006-119194, lowering of a surface temperature of a fixing roller is suppressed by reducing a quantity of heat absorbed in a recording medium from the fixing roller per unit time, thereby fixability can be improved.
In an image forming apparatus, when an external power source is a power source such as a private power generator or electric power is supplied to the image forming apparatus in a form of multiple connection (octopus wiring) together with other OA (Office Automation) equipment, power supply to the image forming apparatus becomes unstable state. Alternatively, impedance of a power supply line becomes large, resulting in the state that voltage varies by the variation of electric current. Furthermore, commercial alternating current source has relatively large variation to rated voltage, depending on power supply circumstance of each country.
When variation of voltage applied to an image forming apparatus is large and low voltage is applied to a heater provided inside a fixing roller, current conduction to the heater is insufficient, and quantity of heat supplied to a core metal of a fixing roller from the heater lacks.
For example, in the image forming apparatus disclosed in JP-A 2004-126192, when voltage applied is lower than rated voltage and current conduction to a heater is initiated, not in the state that applied voltage is lower than rated voltage and a surface temperature of a fixing roller is not sufficiently decreased to room temperature, but in the state that the fixing roller has a heated surface, because the surface temperature of the fixing roller is already high, the measurement time until completion of warming-up operation is short even though the applied voltage is lower than rated voltage. In this case, with respect to the image forming apparatus, storage of heat of the fixing roller is estimated using the measurement time until completion of the warming-up operation, and it is therefore judged that the fixing roller sufficiently stores heat. When job is started in this state, because voltage lower than rated voltage is applied to the heater, the heat storage of the fixing roller is not sufficient, and this may lead to occurrence of poor fixing.
In the image forming apparatus disclosed in JP-A 2006-119194, time interval of conveying a recording medium to a fixing nip region is prolonged or conveying speed of a recording medium is decreased. Therefore, printing speed is decreased, and high speed printing cannot be realized.